Bronx state Sen. Jeff Klein is putting on his hard hat for campaign contributors — at the city’s expense, critics charge.

Klein, the Senate Independent Democratic Conference Leader, is pushing legislation that would allow construction firms to walk off tax-funded jobs amid payment disputes with local governments, The Post has learned.

The main backers of the measure, the Subcontractors Trade Association PAC and Empire State Subcontractors, have pumped $61,200 in donations into Klein’s campaign kitty since 2004, records show.

The bill would allow subcontractors to stop work on public works if additional “unapproved” work orders exceed 10 percent of the original contract.

But Mayor Bloomberg’s office cried foul and issued a strongly worded bill memo opposing the measure as harming students and imperiling public safety.

“The bill will disrupt school construction and rehabilitation because change orders and contract amendments are a routine and expected part of major public-works projects,” said Bloomberg’s state legislative director, Joseph Garba.